Parts made from the state of the art powder metallurgy, i.e., pressed and sintered or infiltrated, have very low impact strengths--typically only 3 to 20 ft. lbs measured by the unnotched Charpy Test. Higher impact strength would enable these low cost methods to be used for higher performance parts that are now made by alternative technologes that are more expensive, i.e., powder metal forging, hot pressing, injection molding, etc.
Copper in iron is known to enable the iron to precipitation harden. Iron also can be hardened by adding carbon and heat treating. The use of carbon and heat treatment is least expensive and virtually the most common way the strength and toughness of steel is controlled.
Prior patent application Ser. No. 755,282, filed July 15, 1985, now U.S. Pat. No. 4,606,768 dated Aug. 19, 1986, assigned to assignee of the present application, describes how to significantly improve impact strength of copper infiltrated steel by assuring the absence of erosion and local porosity (defined statistically in terms of pore volume and maximum pore size). Unnotched Charpy impact strengths as high as 130 ft. lbs at an ultimate tensile strength of 103 ksi have been obtained. High combinations of impact and ultimate tensile strength are sought in many engineering applications. The disclosure of U.S. Pat. No. 4,606,768 is incorporated by reference herein.
State of the art copper infiltration of iron and steel parts uses long infiltration times to insure the most complete infiltration possible and improve tensile strength. Typically, the times range from 30 minutes to 90 minutes, although shorter infiltration times have been reported. For these times there is partial alloying of the copper with the iron due to dissolving and reprecipitation of the iron because of liquid phase sintering as well as solid state diffusion of the coppper into the iron. In the areas where copper and iron are both present, optimizing heat treatment for impact toughness is complicated by both carbon and copper hardening mechanisms operating at the same time.
Several investigators (see U.S. Pat. No. 4,606,768) have attempted to obtain higher combinations of impact strength and tensile strength. Some of these investigators (i.e., Kuroki et al in 1973, Impact Properties of Copper Infiltrated Sintered Iron; Journal Japan Society Powder Metallurgy, July, 1973, Vol 20, pages 71-79) have employed short infiltlration times but were unable to obtain the desirable and large improvements reported herein. The Kuroki et al best combinations for tensile strength and impact strength were 84 ksi and 11 ft. lbs.; 57 ksi and 42 ft. lbs; and 43 ksi and 83 ft. lbs., respectively. The impact strength foot pound data was obtained by conversion from kilogram meters data given in FIG. 2 of the paper, which is based on centimeters squared. The data of highest impact strength was obtained with 100 sec. infiltration time; that of highest tensile strength at 135 min. infiltration time. It is indicated in the Kuroki et al paper that the tests were carried out on notched test pieces which were 8.times.8.times.40 mm in dimension. The notches were described as being U-shaped, having a depth of 1.6 mm with a 1 mm base radius. Conventionally the Charpy Impact Test for powder metal parts is carried out with unnotched test pieces which are 10.times.10 mm in cross-section. In terms of cross-section, the Kuroki et al specimens are non-standard, specifically about 64/100 the size of the parts conventionally tested by the Charpy test.
Notching will have an adverse effect on impact strength. On the other hand, it is well known that the smaller the specimen the better its ductility and toughness. It can be expected that the loss in strength due to notching vs unnotched would be off-set to a greater or lesser degree by the increase in strength due to the use of small-sized specimens. However, it is impossible to correlate the Kuroki et al method of testing with the standard Charpy test. This, plus the lack of certain information in Kuroki et al on the iron powder used, which is believed to be no longer available, or their method of preparation of samples, also makes it impossible to verify the results of Kuroki et al.
One significant fact to note in the Kuroki et al data is that as the impact strength is increased, this is at the expense of tensile strength. They give no data showing improvement at the same time in both tensile strength and impact strength.
One reason for the lower values of Kuroki et al may be improper combinations of carbon content and heat treatment. All of the Kuroki et al data was obtained with carbon-free material.
In this respect, it is well known that the addition of carbon to iron decreases ductility and elongation and would have, as a general rule, an adverse affect on impact strength. It may have been for this reason that Kuroki et al used carbon-free materials.